In a spectrophotometer a beam of light of a selected wavelength or frequency is passed through a medium containing a sample where some of the light is absorbed by the material comprising the sample. Light which passes through the sample medium is received by a light sensitive detector system such as a photometer. Light energy that is not absorbed by the sample results in more light being received by the light detector system. The detector system generates an electrical output signal of a potential or current proportional to the intensity of the light it receives. The output of the light detector system, such as for example one utilizing a photomultiplier tube, is generally an analog current signal proportional to the light intensity received, which thus is proportional to the medium containing the light transmittance of the sample.
The light detector system generally has an amplifier, such as an operational amplifier, to convert the analog current signal from the light detector to an analog d.c. voltage signal. The d.c. voltage signal is processed by additional electronics and applied to a display, such as a chart recorder, to provide a visual and/or permanent record of the sample light transmittance, i.e., absorbance (absorbance =-log (transmittance) log) at a selected light wavelength or through a wavelength scan.
The light beam is generated from a generally white light source through the use of a monochromator. The monochromator provides a monochromatic beam of light having a range of wavelength within a narrow controlled spectral band. This is generally accomplished by dispersing the white light received into a sweeping spectrum of differing wavelengths of light by directing the white light through a prism or reflecting the light from a dispersion grating. The monochromatic light wavelength generated by the monochromator is selected by rotating the prism or grating to direct light of the desired wavelength in the spectrum through a narrow slit or aperture out of the monochromator and into the optical system of the spectrophotometer. Light of undesired wavelengths is not permitted to pass from the monochromator. Thus, by rotating the dispersing element the light spectrum can be moved across the narrow slit to obtain a selected wavelength monochromatic light for application to the sample medium.
The spectral bandwidth of the monochromatic light generated by the monochromator is determined by the width of the slit, the dispersion function of the dispersing element, and the rotational location of the dispersing element relative to the slit. Change in wavelength of the monochromatic light generated, however, usually does not have a linear relationship with change in the angular position of the dispersing element within the monochromator. That is, the wavelength light generated is not a linear function of the dispersing element's change in angle of rotation. Generally, the wavelength of light generated is related to the rotational position of the dispersing element by the following formula: EQU .gamma.=K sin - (1)
where,
.gamma.=wavelength of monochromatic light
K =dispersion constant, e.g. grating constant
.gamma.=angular position of dispersing element
(from a base position)
In order to obtain accurate performance from the spectrophotometer in analysis of a sample it is very important to be able to accurately generate a select and stable beam of monochromatic light for application to the sample. This requires repeatable and precise positioning of the dispersing element in the proper angular position within the monochromator. Manufacture of the instrument components including the dispersing element and its positioning mechanism, and assembly of these components in the instrument, must be performed with very high precision. A complex, highly precise, and expensive mechanism is necessary to direct the dispersing element. Furthermore, the nonlinear relationship between the rotational change of the dispersing element and the change in wavelength of the monochromatic light generated additionally complicates the positioning mechanism design. Prior designs have incorporated complex linkages, precision cams, and specially designed gears to accomplish approximation of linearity between rotation of the dispersing element and wavelength selection in order to obtain accuracy repeatability and ease of use.
With these designs costly and time consuming calibration procedures are necessary to assure proper optical alignment of the assembly and to correct manufacturing variances in the optical elements. Failure to provide either intensive quality control or calibration would often result in spectrophotometer which was unable to accurately analyze, or reproduce accurately the analysis of, the substances for which it was designed.
A need thus exists in the field of scientific instrumentation which utilizes varying wavelength monochromatic light for a select purpose, and with regard to spectrophotometers in particular, to resolve difficulties associated with the manufacture assembly and control of these instruments by providing accuracy and reproducibility in their function with reduced cost of calibration. This need is fulfilled by the invention as set forth herein.